The present invention relates to a fuel cell system, and more particularly to a fuel cell system and method of controlling the same that allows a reduced hydrogen concentration in anode purge gas discharged during anode purge at the start of the fuel cell system.
As a power source that helps to prevent global warming due to exhaust gas, there has been developed a fuel cell that produces electric energy through a chemical reaction between hydrogen and oxygen. In a system with a fuel cell, generally, the cathode is not directly supplied with oxygen but supplied with air containing oxygen. In such fuel cell system, part of the water produced in the cathode and part of the nitrogen contained in the air supplied to the cathode are diffused back through the electrolyte membrane from cathode to anode. Thus, when the fuel cell is started after an interval of time in hours or days, or when the fuel cell is continued to operate, the concentration of hydrogen as anode gas may be decreased relatively due to the increased concentration of water and nitrogen in the anode, which may result in reduced efficiency of electric power generation. To prevent this, generally, purging of the anode (hereinafter referred to as anode purge) is carried out by discharging water and nitrogen in the anode with hydrogen as anode gas. In such anode purge, anode purge gas is diluted with cathode off-gas or using a diluter before being discharged into the atmosphere so that hydrogen concentration in the anode purge gas is decreased to a safe level.
In a conventional fuel sell system disclosed in Japanese Unexamined Patent Application Publication No. 2006-99989, a hydrogen exhaust passage, a hydrogen purge valve and a hydrogen dilution fan are provided on the anode outlet side. In the anode purge when the fuel cell is started or stopped, the hydrogen purge valve is intermittently opened for a short time, and the hydrogen dilution fan is operated depending on the hydrogen concentration in the anode purge gas discharged from the hydrogen exhaust passage so as to diffuse and dilute the anode purge gas with air. As a result, the hydrogen concentration in the anode purge gas is decreased below a predetermined level.
In another conventional fuel cell system disclosed in Japanese Unexamined Patent Application Publication No. 2008-235205, anode purge gas is discharged into the atmosphere through a diluter and an exhaust pipe provided on the anode outlet side. The diluter is connected to a cathode off-gas passage through which cathode off-gas (air) is supplied to the diluter. The fuel cell system further includes a heat medium passage through which cooling water for maintaining the fuel cell at an appropriate temperature is circulated, a radiator provided in the heat medium passage, and a radiator fan for blowing air through the radiator. Airflow generated by the radiator fan is delivered to a position adjacent to the outlet of the exhaust pipe. Thus, the airflow from the radiator fan forces the anode purge gas diluted with the cathode off-gas and discharged from the exhaust pipe into the atmosphere to be diffused, resulting in reduction of hydrogen concentration in the anode purge gas below a predetermined level.
In the fuel cell system disclosed in the publication No. 2006-99989, however, the hydrogen dilution fan additionally provided results in increased size of the system and increased number of parts of the system. On the other hand, in the fuel cell system disclosed in the publication No. 2008-235205, though the use of existing radiator fan helps to prevent increased size of the system and increased number of parts of the system, diffusion of anode purge gas into the atmosphere may not be accomplished sufficiently in the anode purge at the start of the fuel cell. This is because, in general, the radiator fan is started to operate when the cooling water reaches a given temperature and, therefore, the radiator fan is at a stop or operated at a low speed in a condition that only a short time has elapsed after the start of the fuel cell and the amount of heat generated in the fuel cell is still small. In addition, the cathode off-gas (air) for diluting anode purge gas is not supplied sufficiently just after the start of the fuel cell, which may result in insufficient dilution of anode purge gas with the cathode off-gas.
The present invention is directed to providing a fuel cell system that allows hydrogen concentration in anode purge gas to be reduced efficiently and sufficiently during anode purge at a start of the fuel cell system.